In the art it is desirable to produce elastomeric compounds exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents, followed by vulcanization. Such elastomers, when compounded, fabricated and vulcanized into components for constructing articles such as tires, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomeric compound refers to the difference between the energy applied to deform an article made from the elastomeric compound and the energy released as the elastomeric compound returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn,
In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system.
Previous attempts at preparing reduced hysteresis products have focused upon increased interaction between the elastomer and the compounding materials such as carbon black, including high temperature mixing of the filler-rubber mixtures in the presence of selectively-reactive promoters to promote compounding material reinforcement, surface oxidation of the compounding materials, chemical modifications to the terminal end of polymers using 4,4'-bis(dimethylamino)benzophenone (Michler's ketone), tin coupling agents and the like and, surface grafting.
It has also been recognized that carbon black, employed as a reinforcing filler in rubber compounds, should be well dispersed throughout the rubber in order to improve various physical properties. One example of this recognition is provided in published European Pat. Appln. EP 0 316 255 A2 which discloses a process for end capping polydienes by reacting a metal terminated polydiene with a capping agent such as a halogenated nitrile, a heterocyclic aromatic nitrogen-containing compound or an alkyl benzoate. Additionally, the application discloses that both ends of the polydiene chains can be capped with polar groups by utilizing functionalized initiators, such as dialkylamino lithium compounds. However, results from such initiation show that no improvement in hysteresis is achieved when the elastomeric compositions are filled with carbon black.
Various organolithium polymerization initiators are also known in the art. U.S. Pat. No. 3,439,049, owned by the Assignee of record, discloses an organolithium initiator prepared from a halophenol in a hydrocarbon medium.
U.S. Pat. No. 4,015,061 is directed toward amino-functional initiators which polymerize diene monomers to form mono- or di-primary aryl amine-terminated diene polymers upon acid hydrolysis. U.S. Pat. No. 4,914,147 discloses terminal modifying agents including dialkylamino-substituted aromatic vinyl compounds such as N,N'-dimethylamino benzophenone and p-dimethylamino styrene, in rubber compositions having reduced hysteresis characteristics. In U.S. Pat. No. 4,894,409, an amino group-containing monomer, such as 2-N,N-dimethylaminostyrene is polymerized to form an amino group-containing diene based polymer.
Other patents owned by the assignee of record, directed to amine-containing polymerization initiators include U.S. Patent Nos. 5,238,893; 5,274,106; 5,329,005; 5,332,810; 5,393,721; 5,496,940; 5,519,086; 5,521,309; 5,523,371; and 5,552,473. Lawson, et al., in Anionic Polymerization of Dienes Using Homogeneous Lithium Amide (N-Li) Initiators, ACS Preprint, Polymer Division, 37 (2) 1996, at page 728, disclosed that, for carbon black-filled compounds, only cyclic amino lithium initiators of a certain size provided elastomeric compounds exhibiting reduced hysteresis. The carbon black-filled elastomeric compounds containing aliphatic amino initiators did not reduce hysteresis.
Precipitated silica has been increasingly used as a reinforcing particulate filler in carbon black-filled rubber components of tires and mechanical goods. Silica-loaded rubber stocks, however, exhibit relatively poor resilience and, thus, increased hysteresis.
The present invention provides initiators for anionic polymerization which become incorporated into the elastomer chain, as well as various terminators for the polymerization, and produce functional groups which greatly improve the dispersability of silica filler throughout the elastomeric composition during compounding. As will be described hereinbelow, these initiators and terminators both contain amine groups. The initiator further contains a fugitive lithium moiety.
It has heretofore been unknown to utilize a combination of both amine producing initiators and terminators to increase dispersion of silica filler in diene polymer and copolymer elastomeric compounds, and to thereby reduce the hysteresis of the cured compounds.